This study will investigate the involvement of 5-stereonergic mechanisms in regulating delay discounting in opioid-dependent patients. In the first step we will determine the rate of discounting for hypothetical money and heroin in injection drug users in outpatient treatment for heroin dependence. In a second step we will determine whether serotonin is involved in the differences observed by administering a hormonal challenge test with the serotonergic probe citalopram to needle-sharers and non-sharers. The proposed research may provide valuable information regarding the biological basis of addiction.